This invention relates to color cathode ray tubes (CRTs) and, more particularly, to miniature color CRTs in which the target is addressed by a single electron beam (e-beam). These CRTs are suitable for projection display systems.
As discussed by S. Sherr in a book entitled, Electronic Displays, John Wiley and Sons, (1979), the color CRT has undergone considerable development because of its extensive use in home entertainment television. Although the designs have been satisfactory for that application, they are not adequate for all information display systems, particularly projection display systems.
Color CRTs can be divided into two major categories: those which utilize three e-beams, one to generate each primary color; and those which utilize a single e-beam to generate all of the primary colors. The most successful color CRT, and that adopted by the majority of the television manufacturers, utilizes the three beam technique to address color triads on a phosphor screen. A shadow mask, consisting of a plate having circular apertures, is interposed between the screen and the electron guns, which are arranged side by side. The three beams follow slightly different paths which converge to a focus on the shadow mask aperture. The arrangement is designed so that the beam corresponding to the desired color strikes only the phosphor dot of the triad producing that color. All three beams are deflected together with a single yoke, and the electrostatic focus elements for the three guns are connected in parallel so that a single focus control is sufficient.
Even if perfect alignment of the masks and phosphor triads is assumed, the CRT is still subject to certain limitations of resolution and luminance according to Sherr. The resolution restriction arises from the necessity to align the mask apertures and the phosphor dot triads, so that the mask aperture size controls the obtainable resolution. In addition, misalignment and misregistration of the three beams leads to loss of purity for colors produced by combinations of the primary colors as well as some reduction in luminance due to a smaller part of the beams passing through the apertures. Moreover, dynamic focusing becomes very important and adds to the cost and long-term instability of the focus.
One improvement in color CRTs has been the in-line gun in which three electron guns are placed in a line perpendicular to the axis and the shadow mask grid. The resulting beams are directed through a striped grid onto a screen consisting of parallel stripes of color phosphors in alternating red, green, and blue triplets. Since a single lens with a large diameter is used, aberrations are kept smaller, and smaller spot sizes may be achieved compared to conventional guns. The in-line guns also simplify convergence, and the transmission of the stripe apertures is considerably better than that of the conventional shadow mask apertures.
Another attempt to improve on the performance of the shadow mask color CRT involves the use of a single e-beam to address the color triads. Consequently, some form of beam indexing is used to determine the precise position of the scanning beam reative to the triad. In the pilot beam version, described at page 123 by Sherr, the electron gun contains a single cathode and means for splitting up the electrons into two beams. The primary beam generates color in the usual fashion, and the pilot beam is used to determine the position of the primary beam. The color phosphors are parallel stripes, and behind the red phosphor stripes are secondary emission index stripes. A particular frequency mixing scheme is used to determine when the primary color beam is at the red phosphor stripe, with the other colors coming at fixed periods in relation to that time. The system, however, imposes too many severe requirements on the structure of the CRT to be practical and has been abandoned after several years of intense development.
Another beam index tube has been developed using UV phosphor index stripes in place of the secondary emission index stripes described above. Once again, the target includes blue, red, and green striped triads with the index stripes located between alternate pairs of color stripes. A photomultiplier is used to detect the UV emission generated when the e-beam is incident on an index stripe. Again, this index signal must be mixed with a chrominance signal in a manner similar to the secondary emission version to produce a chrominance component of the video signal applied to the CRT grid. This approach has the advantage that only a single beam is needed and that the index current becomes zero when the beam leaves the index stripe. However, according to Sherr, most of the other problems found in the construction of the pilot beam tube remain.
Another approach to the production of a color image on a CRT, without requiring either multiple beams, masks or index stripes, is the beam penetration tube. This color CRT is based on the principle that the depth of penetration of an e-beam into a phosphor is proportional to the difference between the square of the initial electron energy and the square of the remaining electron energy after penetration to a particular depth. Thus, a single e-beam and a multilayer phosphor may be used to generate the different colors, eliminating the need for masks or grids to ensure that the proper beam hits the correct phosphor. This CRT, however, is limited by the circuit complexity required to switch high voltages at relatively high rates of speed. Normal accelerating voltages range from 6 kV for red to about 12 kV for green. In order to switch colors at television rates, it is necessary to change the voltage in about 100 nsec (assuming standard sweep rates and horizontal resolution, with a single sweep time of 60 .mu.sec, and 480 resolution elements in one sweep time). The actual number is not very important since switching 6 kV into a capacitance of 100 pF or larger is a formidable task requiring high power radar techniques according to Sherr. Such tubes are proving to be useful for color graphics where the color of interest is changed only infrequently. However, despite intense development, they are not useful for color television.